1. Field of the Invention
This invention relates to compositions of ethylene-vinyltriethoxysilane copolymers containing relatively high levels of particulate fillers which are resistant to premature crosslinking. Compositions wherein an ethylene polymer having rubber-like properties is included with the silane copolymer and which upon cure exhibit superior physical properties are also disclosed. The invention also relates to an improved method for producing filled crosslinked silane copolymer compositions whereby problems generally associated with processing such materials are minimized.
2. Description of the Prior Art
Crosslinking ethylene polymers to improve their mechanical strength, heat resistance and other properties is well known in the art. One method of crosslinking ethylene polymers involves incorporating alkoxysilane functionality into the polymer structure--either by grafting unsaturated alkoxysilanes onto ethylene polymers or by direct copolymerization of ethylene with unsaturated alkoxysilanes.
The alkoxysilane groups form crosslinks either by the action of heat and/or mechanical working or by hydrolysis/condensation reactions. Considering only the reaction of a single alkoxy group of a silane moiety the hydrolysis/condensation reaction would proceed according to the following schemes: ##STR1##
Crosslinkable ethylene polymers prepared by grafting unsaturated alkoxysilanes to ethylene polymers are disclosed in U.S. Pat. No. 3,646,155. Crosslinkable ethylene polymers prepared by copolymerizing ethylene with an unsaturated alkoxysilane are disclosed in U.S. Pat. Nos. 3,225,018 and 3,392,156.
One of the major limitations of crosslinking ethylene polymers which contain alkoxysilane functional groups is that incidental exposure to moisture can initiate the crosslinking reaction prematurely. Significant premature crosslinking can occur even in the absence of silanol condensation catalysts which are typically employed to facilitate the hydrolysis and condensation reactions. Premature crosslinking adversely affects (lowers) the melt index of the ethylene copolymer and, if this occurs to a significant degree, will result in poor processability and unsatisfactory extrudate surface appearance. In extreme cases, extrusion of the ethylene copolymer compositions becomes essentially impossible.
The problem of premature crosslinking is particularly troublesome with filled compositions since particulate fillers can contain substantial amounts of moisture. This moisture is released during the blending operation, batch or continuous, typically mixing on a mill, in a Banbury mixer, or in a compounding extruder at an elevated temperature, and hydrolyzes at least some of the alkoxy groups of the alkoxysilane. Some condensation may also occur during the mixing. Furthermore hydrolysis and condensation will continue even after the mixing is completed as evidenced by a further deterioration of the physical properties of the filled composition upon storage and conveniently followed by monitoring the change in melt index The problems associated with highly filled ethylene-vinylalkoxysilane copolymers are widely recognized throughout the industry and is described in European Patent Application 89310667.4 published Apr. 25, 1990.
Several solutions to this problem have been suggested in the prior art; however, none of the proposed solutions have resulted in a totally satisfactory commercial material and/or process to date. One method disclosed in the above-identified European Patent Application requires blending an ethylene-vinyltrialkoxysilane copolymer with a masterbatch containing a second polymer, the filler and silanol condensation catalyst immediately prior to extrusion. Even with this method it is recommended that the copolymer and masterbatch containing the filler be dried before extrusion. It further requires that if the compositions are to be stored, they be stored separately. It does not address or solve the problem of storage of ethylene-vinylalkoxysilane copolymers in combination with fillers.
Another method disclosed in U.S. Pat. No. 4,732,939 involves blending a silane-grafted polymer and an ethylenic polymer consisting mainly of an ethylene-alpha-olefin copolymer and filler. Dilution of the crosslinkable polymer lessens the effects of premature crosslinking and thus renders the filled blends processable. The absence of a free radical generator and catalytic accelerator, however, limits the ability of these blends to achieve satisfactory levels of crosslinking and develop the physical properties necessary for some critical applications. Also, the blend is limited to densities from 0.890 to 0.915 g/cm.sup.3 which also limits the utility of these products. For many applications, higher density materials are desirable.
Yet another method disclosed in Japanese Patent Applications 56,163,143, 59,217,750 and 62,184,707 requires that the filled moisture crosslinkable ethylene polymer system be extruded in the absence of condensation catalyst. The extruded product is subsequently immersed in a solution or emulsion of the catalyst. Such a process adds an additional step to the curing operation and makes the practice of moisture crosslinking quite cumbersome.
The addition of water scavengers to the filled moisture crosslinkable polymer systems to prevent crosslinking has also been suggested. Such methods are disclosed in U.S. Pat. No 4,707,520 and European Patent Application 87302899.7 published Nov. 19, 1987.